Catalytic properties and sensitivity to tentoxin of Chlamydomonas reinhardtii ATP synthases changed in codon 83 of atpB by site-directed mutagenesis

Functional characterization of Chlamydomonas reinhardtii with alterations in the atpE gene

The FUD17 strain of Chlamydomonas reinhardtii is a photosynthesis-deficient, acetate-requiring mutant with a defect in the chloroplast atpE gene, which codes for the epsilon subunit of the chloroplast ATP synthase. In this work, the FUD17 mutant was examined in relation to other known ATP synthase mutants as an initial step toward using this strain to generate altered versions of the atpE gene for site-directed mutagenesis of the epsilon subunit. The FUD17 strain grows well and is normally pigmented in the dark (heterotrophic conditions), but cannot grow autotrophically in the light, even when media are supplemented with acetate. Under heterotrophic conditions, it shows no accumulation of the epsilon subunit, and much lower levels of the alpha and beta subunits of the chloroplast ATP synthase. FUD17 shows no light-dependent oxygen evolution and shows a strong, light-dependent alteration in its chlorophyll fluorescence. These results show that FUD17 possesses similar characteristics to other ATP synthase mutants and fails to express an assembled ATP synthase complex on its thylakoid membrane. A preliminary attempt at site-directed mutagenesis is described which produced a slightly truncated form of the epsilon subunit, which is expressed normally in the cell.

Complete inhibition of the tentoxin-resistant F1-ATPase from Escherichia coli by the phytopathogenic inhibitor tentoxin after substitution of critical residues in the alpha - and beta -subunit

Substitution of critical residues in the alpha- and beta-subunit can turn the typically resistant ATP synthase from the bacterium Escherichia coli into an enzyme showing high sensitivity to the phytopathogenic inhibitor tentoxin, which usually affects only certain sensitive plant species. In contrast to recent results obtained with the thermophilic F(1) (Groth, G., Hisabori, T., Lill, H., and Bald, D. (2002) J. Biol. Chem. 277, 20117-20119), substitution of a critical serine in the beta-subunit (betaSer(59)), which is supposed to provide an important intermolecular hydrogen bond in the binding site, was not sufficient on its own for conferring tentoxin sensitivity to the E. coli F(1) complex. Superimposition of the chloroplast F(1)-tentoxin inhibitor complex on a homology model of the E. coli F(1) complex provided detailed information on the critical residues in the alpha-subunit of the binding cleft and allowed us to model the binding site according to the steric requirements of the inhibitor. Substitution of the highly conserved residue alphaLeu(64) seems to be most important for allowing access of the inhibitor to the binding site. Combining this substitution with either additional replacements in the alpha-subunit (Q49A, L95A, E96Q, I273M) or the replacement of Ser(59) in the beta-subunit enhanced the sensitivity to the inhibitor and resulted in a complete inhibition of the E. coli F(1)-ATPase by the plant-specific inhibitor tentoxin.

Rebuilt 3D structure of the chloroplast f1 ATPase-tentoxin complex

The F1 part of the chloroplast H+ adenosine triphosphate (ATP)-synthase (CF1) strongly interacts with tentoxin, a natural fungous cyclic tetrapeptide known to inhibit the chloroplast enzyme and not the mammalian mitochondrial enzyme. Whereas the synthesis or the hydrolysis of ATP requires the stepwise rotation of the protein rotor gamma within the (alphabeta)3 crown, only one molecule of tentoxin is needed to fully inhibit the complex. With the help of an original homology modeling technique, based on robust distance geometry protocols, we built a tridimensional model of the alpha3beta3gamma CF1) subcomplex (3200 esidues), in which we introduced three different nucleotide occupancies to check their possible influence on the tentoxin binding site. Simultaneous comparison of three available high-resolution X-ray structures of F1, performed with a local structural alignment search tool, led to characterizing common structural blocks and the distorsions experienced by the complex during the catalytic turnover. The common structural blocks were used as a starting point of the spinach CF1 structure rebuilding. Finally, tentoxin was docked into its putative binding site of the reconstructed structure. The docking method was initially validated in the mitochondrial enzyme by its ability to relocate nucleotides into their original position in the crystal. Tentoxin binding was found possible to the two alpha/beta interfaces associated with the empty and adenosine diphosphate (ADP)-loaded catalytic sites, but not to the one associated with the ATP-loaded site. These results suggest a mechanism of CF1 inhibition by one molecule of tentoxin, by the impossibility of the alpha/beta interface bearing tentoxin to pass through the ATP-loaded state.

Substitution of a single amino acid switches the tentoxin-resistant thermophilic F1-ATPase into a tentoxin-sensitive enzyme

In contrast to the homologous bacterial and mitochondrial enzymes the chloroplast F(1)-ATPase (CF(1)) is strongly affected by the phytopathogenic inhibitor tentoxin. Based on structural information obtained from crystals of a CF(1)-tentoxin co-complex (Groth, G. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 3464-3468) we have replaced residues betaSer(66) and alphaArg(132) in the alpha(3)beta(3)gamma subcomplex of the thermophilic F(1)-ATPase from Bacillus PS3 by the corresponding residues of the chloroplast ATPase to confer tentoxin sensitivity to the thermophilic enzyme. The mutation alphaArg(132) --> Pro, proposed to relieve steric constraints on tentoxin binding, did not have any significant effect. However, mutation betaSer(66) --> Ala, predicted to provide a crucial hydrogen bond with the inhibitor, resulted in tentoxin inhibition of ATP hydrolysis comparable with the situation found with the chloroplast enzyme.

Structure of spinach chloroplast F1-ATPase complexed with the phytopathogenic inhibitor tentoxin

Tentoxin, a natural cyclic tetrapeptide produced by phytopathogenic fungi from the Alternaria species affects the catalytic function of the chloroplast F(1)-ATPase in certain sensitive species of plants. In this study, we show that the uncompetitive inhibitor tentoxin binds to the alphabeta-interface of the chloroplast F(1)-ATPase in a cleft localized at betaAsp-83. Most of the binding site is located on the noncatalytic alpha-subunit. The crystal structure of the tentoxin-inhibited CF(1)-complex suggests that the inhibitor is hydrogen bonded to Asp-83 in the catalytic beta-subunit but forms hydrophobic contacts with residues Ile-63, Leu-65, Val-75, Tyr-237, Leu-238, and Met-274 in the adjacent alpha-subunit. Except for minor changes around the tentoxin-binding site, the structure of the chloroplast alpha(3)beta(3)-core complex is the same as that determined with the native chloroplast ATPase. Tentoxin seems to act by inhibiting inter-subunit contacts at the alphabeta-interface and by blocking the interconversion of binding sites in the catalytic mechanism.

CHLAMYDOMONAS AS A MODEL ORGANISM

The unicellular green alga Chlamydomonas offers a simple life cycle, easy isolation of mutants, and a growing array of tools and techniques for molecular genetic studies. Among the principal areas of current investigation using this model system are flagellar structure and function, genetics of basal bodies (centrioles), chloroplast biogenesis, photosynthesis, light perception, cell-cell recognition, and cell cycle control. A genome project has begun with compilation of expressed sequence tag data and gene expression studies and will lead to a complete genome sequence. Resources available to the research community include wild-type and mutant strains, plasmid constructs for transformation studies, and a comprehensive on-line database.

Formation and properties of hybrid photosynthetic F1-ATPases. Demonstration of different structural requirements for stimulation and inhibition by tentoxin

A hybrid ATPase composed of cloned chloroplast ATP synthase beta and gamma subunits (betaC and gammaC) and the cloned alpha subunit from the Rhodospirillum rubrum ATP synthase (alphaR) was assembled using solubilized inclusion bodies and a simple single-step folding procedure. The catalytic properties of the assembled alpha3Rbeta3CgammaC were compared to those of the core alpha3Cbeta3CgammaC complex of the native chloroplast coupling factor 1 (CF1) and to another recently described hybrid enzyme containing R. rubrum alpha and beta subunits and the CF1 gamma subunit (alpha3Rbeta3RgammaC). All three enzymes were similarly stimulated by dithiothreitol and inhibited by copper chloride in response to reduction and oxidation, respectively, of the disulfide bond in the chloroplast gamma subunit. In addition, all three enzymes exhibited the same concentration dependence for inhibition by the CF1 epsilon subunit. Thus the CF1 gamma subunit conferred full redox regulation and normal epsilon binding to the two hybrid enzymes. Only the native CF1 alpha3Cbeta3CgammaC complex was inhibited by tentoxin, confirming the requirement for both CF1 alpha and beta subunits for tentoxin inhibition. However, the alpha3Rbeta3CgammaC complex, like the alpha3Cbeta3CgammaC complex, was stimulated by tentoxin at concentrations in excess of 10 microm. In addition, replacement of the aspartate at position 83 in betaC with leucine resulted in the loss of stimulation in the alpha3Rbeta3CgammaC hybrid. The results indicate that both inhibition and stimulation by tentoxin require a similar structural contribution from the beta subunit, but differ in their requirements for alpha subunit structure.

The structure of the chloroplast F1-ATPase at 3.2 A resolution

The structure of the F(1)-ATPase from spinach chloroplasts was determined to 3.2 A resolution by molecular replacement based on the homologous structure of the bovine mitochondrial enzyme. The crystallized complex contains four different subunits in a stoichiometry of alpha(3)beta(3)gammaepsilon. Subunit delta was removed before crystallization to improve the diffraction of the crystals. The overall structure of the noncatalytic alpha-subunits and the catalytic beta-subunits is highly similar to those of the mitochondrial and thermophilic subunits. However, in the crystal structure of the chloroplast enzyme, all alpha- and beta-subunits adopt a closed conformation and appear to contain no bound adenine nucleotides. The superimposed crystallographic symmetry in the space group R32 impaired an exact tracing of the gamma- and epsilon-subunits in the complex. However, clear electron density was present at the core of the alpha(3)beta(3)-subcomplex, which probably represents the C-terminal domain of the gamma-subunit. The structure of the spinach chloroplast F(1) has a potential binding site for the phytotoxin, tentoxin, at the alphabeta-interface near betaAsp(83) and an insertion from betaGly(56)-Asn(60) in the N-terminal beta-barrel domain probably increases the thermal stability of the complex. The structure probably represents an inactive latent state of the ATPase, which is unique to chloroplast and cyanobacterial enzymes.

Hybrid Rhodospirillum rubrum F(0)F(1) ATP synthases containing spinach chloroplast F(1) beta or alpha and beta subunits reveal the essential role of the alpha subunit in ATP synthesis and tentoxin sensitivity

Trace amounts ( approximately 5%) of the chloroplast alpha subunit were found to be absolutely required for effective restoration of catalytic function to LiCl-treated chromatophores of Rhodospirillum rubrum with the chloroplast beta subunit (Avital, S., and Gromet-Elhanan, Z. (1991) J. Biol. Chem. 266, 7067-7072). To clarify the role of the alpha subunit in the rebinding of beta, restoration of catalytic function, and conferral of sensitivity to the chloroplast-specific inhibitor tentoxin, LiCl-treated chromatophores were analyzed by immunoblotting before and after reconstitution with mixtures of R. rubrum and chloroplast alpha and beta subunits. The treated chromatophores were found to have lost, in addition to most of their beta subunits, approximately a third of the alpha subunits, and restoration of catalytic activity required rebinding of both subunits. The hybrid reconstituted with the R. rubrum alpha and chloroplast beta subunits was active in ATP synthesis as well as hydrolysis, and both activities were completely resistant to tentoxin. In contrast, a hybrid reconstituted with both chloroplast alpha and beta subunits restored only a MgATPase activity, which was fully inhibited by tentoxin. These results indicate that all three copies of the R. rubrum alpha subunit are required for proton-coupled ATP synthesis, whereas for conferral of tentoxin sensitivity at least one copy of the chloroplast alpha subunit is required together with the chloroplast beta subunit. The hybrid system was further used to examine the effects of amino acid substitution at position 83 of the beta subunit on sensitivity to tentoxin.

Refolding of recombinant alpha and beta subunits of the Rhodospirillum rubrum F(0)F(1) ATP synthase into functional monomers that reconstitute an active alpha(1)beta(1)-dimer

The alpha subunit from the Rhodospirillum rubrum F(0)F(1) ATP synthase (RrF(1)alpha) was over-expressed in unc operon-deleted Escherichia coli strains under various growth conditions only in insoluble inclusion bodies. The functional refolding of urea-solubilized RrF(1)alpha was followed by measuring its ability to stimulate the restoration of ATP synthesis and hydrolysis in beta-less R. rubrum chromatophores reconstituted with pure native or recombinant RrF(1)beta [Nathanson, L. & Gromet-Elhanan, Z. (1998) J. Biol. Chem. 273, 10933-10938]. The refolding efficiency was found to increase with decreasing RrF(1)alpha concentrations and required high concentrations of MgATP, saturating approximately 60% when 50 microgram protein.mL(-1) were refolded in presence of 50 mM MgATP. Size-exclusion HPLC of such refolded RrF(1)alpha revealed a 50-60% decrease in its aggregated form and a parallel appearance of its monomeric peak. RrF(1)beta refolded under identical conditions appeared almost exclusively as a monomer. This procedure enabled the isolation of large amounts of a stable RrF(1)alpha monomer, which stimulated the restoration of ATP synthesis and hydrolysis much more efficiently than the refolded alpha mixture, and bound ATP and ADP in a Mg-dependent manner. Incubation of both RrF(1)alpha and beta monomers, which by themselves had no ATPase activity, resulted in a parallel appearance of activity and assembled alpha(1)beta(1)-dimers, but showed no formation of alpha(3)beta(3)-hexamers. The RrF(1)-alpha(1)beta(1)-ATPase activity was, however, very similar to the activity observed in isolated native chloroplast CF(1)-alpha(3)beta(3), indicating that these dimers contain only the catalytic nucleotide-binding site at their alpha/beta interface. Their inability to associate into an alpha(3)beta(3)-hexamer seems therefore to reflect a much lower stability of the noncatalytic RrF(1) alpha/beta interface.

Kinetic analysis of tentoxin binding to chloroplast F1-ATPase. A model for the overactivation process

The mechanism of action of tentoxin on the soluble part (chloroplast F1 H+-ATPase; CF1) of chloroplast ATP synthase was analyzed in the light of new kinetic and equilibrium experiments. Investigations were done regarding the functional state of the enzyme (activation, bound nucleotide, catalytic turnover). Dialysis and binding data, obtained with 14C-tentoxin, fully confirmed the existence of two tentoxin binding sites of distinct dissociation constants consistent with the observed Kinhibition and Koveractivation. This strongly supports a two-site model of tentoxin action on CF1. Kinetic and thermodynamic parameters of tentoxin binding to the first site (Ki = 10 nM; kon = 4.7 x 10(4) s-1.M-1) were determined from time-resolved activity assays. Tentoxin binding to the high affinity site was found independent on the catalytic state of the enzyme. The analysis of the kinetics of tentoxin binding on the low affinity site of the enzyme showed strong evidence for an interaction between this site and the nucleotide binding sites and revealed a complex relationship between the catalytic state and the reactivation process. New catalytic states of CF1 devoid of epsilon-subunit were detected: a transient overstimulated state, and a dead end complex unable to bind a second tentoxin molecule. Our experiments led to a kinetic model for the reactivation phenomenon for which rate constants were determined. The implications of this model are discussed in relation to the previous mechanistic hypotheses on the effect of tentoxin.

Demonstration of thermal dissipation of absorbed quanta during energy-dependent quenching of chlorophyll fluorescence in photosynthetic membranes

When plant leaves or chloroplasts are exposed to illumination that exceeds their photosynthetic capacity, photoprotective mechanisms such as described by the energy-dependent (non-photochemical) quenching of chlorophyll fluorescence are involved. The protective action is attributed to an increased rate constant for thermal dissipation of absorbed quanta. We applied photoacoustic spectroscopy to monitor thermal dissipation in spinach thylakoid membranes together with simultaneous measurement of chlorophyll fluorescence in the presence of inhibitors of opposite action on the formation of delta pH across the thylakoid membrane (tentoxin and nigericin/valinomycin). A linear relationship between the appearance of fluorescence quenching during formation of the delta pH and the reciprocal variation of thermal dissipation was demonstrated. Dicyclohexylcarbodiimide, which is known to prevent protonation of the minor light-harvesting complexes of photosystem II, significantly reduced the formation of fluorescence quenching and the concurrent increase in thermal dissipation. However, the addition of exogenous ascorbate to activate the xanthophyll de-epoxidase increased non-photochemical fluorescence quenching without affecting the measured thermal dissipation. It is concluded that a portion of energy-dependent fluorescence quenching that is independent of de-epoxidase activity can be readily measured by photoacoustic spectroscopy as an increase in thermal deactivation processes.

Mutagenesis of beta-Glu-195 of the Rhodospirillum rubrum F1-ATPase and its role in divalent cation-dependent catalysis

We introduced mutations at the fully conserved residue Glu-195 in subunit beta of Rhodospirillum rubrum F1-ATPase. The activities of the expressed wild type (WT) and mutant beta subunits were assayed by following their capacity to assemble into the earlier prepared beta-depleted, membrane-bound R. rubrum enzyme (Philosoph, S., Binder, A., and Gromet-Elhanan, Z. (1977) J. Biol. Chem. 252, 8742-8747) and to restore ATP synthesis and/or hydrolysis activity. All three mutations, beta-E195K, beta-E195Q, and beta-E195G, were found to bind as the WTbeta into the beta-depleted enzyme. They restored between 30 and 60% of the WT restored photophosphorylation activity and 16, 45, and 105%, respectively of the CaATPase activity. The mutants required, however, much higher concentrations of divalent cations and could not restore any significant MgATPase or MnATPase activities. Only beta-E195G could restore some of these activities when assayed in the presence of 100 mM sulfite and high MgCl2 or MnCl2 concentrations. These results suggest that the observed difference in restoration of ATP synthesis and CaATPase, as compared with MgATPase and MnATPase, can be due to the tight regulation of the last two activities, resulting in their inhibition at cation/ATP ratios above 0.5. The R. rubrum F1beta-E195 is equivalent to the mitochondrial F1beta-E199, which points into the tunnel leading to the F1 catalytic nucleotide binding sites (Abrahams, J. P., Leslie, A. G. W., Lutter, R., and Walker, J. E. (1994) Nature 370, 621-628). Our findings indicate that this residue, although not an integral part of the F1 catalytic sites, affects divalent cation binding and release of inhibitory MgADP, suggesting its participation in the interconversion of the F1 catalytic sites between different conformational states.

The C. reinhardtii CF1 with the mutation betaT168S has high ATPase activity

We have generated the mutation T168S in the beta subunit of the chloroplast ATP synthase complex of Chlamydomonas reinhardtii by site directed mutagenesis and chloroplast transformation. CF1 and the alpha3beta3gamma complex of this mutant strain were isolated and their enzymatic activities were characterized and compared to those of the corresponding wild type complexes. Without activation the mutant CF1 exhibits MgATPase activity with at least 10 times higher rates than the wild type enzyme. The MgATPase activity could be stimulated to some extent by methanol, but less by ethanol and octylglucoside. The alpha3beta3gamma complex had an even higher MgATPase activity, which was only slightly enhanced by ethanol or methanol. The ATPase activities of the mutant complexes, like those of the wild type complexes, displayed a sharp concentration optimum for Mg2+. Free ADP inhibited neither the mutant nor the wild type ATPase significantly. Azide, which strongly inhibited the ATPase activity of the wild type enzyme, inhibited the mutant enzyme only at an about 30 times higher concentration suggesting that the mutation T168S prevents trapping of a tightly bound MgADP by a catalytic site that regulates chloroplast ATPase activity. The mutant cells grew photoautotrophically at a growth rate of about 50%. Similar to the wild type the cells survived on minimal medium in the dark. Under heterotrophic conditions with acetate as energy and carbon source the mutant cells grew much faster than the wild type cells, but the chlorophyll content per cell decreased dramatically.

Three kinds of binding site for tentoxin on isolated chloroplast coupling factor 1

Tentoxin binding on chloroplast coupling factor 1 (CF1) was studied using a centrifugation column method followed by HPLC analysis. From non-linear regression analysis of the results, the presence of three types of binding site with the following Kd values was deduced: 6.9x10(-8) M (first site), 1.4x10(-5) M (second site), and 6.3x10(-3) M (third site). The binding of one tentoxin inhibits, that of two tentoxins moderately restores, and that of three tentoxins greatly stimulates the ATPase activity of CF1. The forward rate constant of the binding of tentoxin on the first site was 6.3x10(3) M-1 s-1.